An image pick-up apparatus such as a camera is typically includes an auto focusing feature to the convenience of affecting the focusing on the subject automatically. To achieve the automatic focusing, a through-the-lens (TTL) auto focusing method, which utilizes the light entering through a photographing lens, is often used. An auto focus module may detect whether the subject is in focus or not by employing the contrast detection method or the phase difference detection method. In the contrast detection method, repeated calculations of the contrast of a portion of an image (mainly, a center portion) are made while moving the lens with the subject being deemed in focus when the contrast reaches a maximum value. The contrast detection method is generally used in video cameras and compact digital cameras. According to the phase difference detection method, the incoming light received through a lens is divided into a pair of light beams, which are compared with each other to determine whether the subject is in focus. The phase difference detection method may be thought as operating in a similar way as the manual focusing through a split image screen, for example. Most of film SLR cameras and digital SLR cameras currently available use the phase difference detection method.
In the above described TTL auto focusing methods employed in a SLR camera, the light entering through a photographing lens is made incident on a main mirror that is partially reflective to reflect some of the light toward the view finder while allowing the remainder portion of the light to pass therethrough. The light passing through the main mirror is then reflected in the direction of an auto focus module off a sub-mirror positioned behind the main mirror. The main mirror and the sub-mirror of a conventional image pick-up apparatus are arranged to operate in unison so that a portion less than the total amount of the available light becomes incident on the auto focus module, which makes it difficult to focus on the subject under a low light condition. Further in such conventional image pick-up apparatus that provide a live view mode, which mode provides a display, e.g., on a liquid crystal display (LCD) screen, of the image acquired from the light incident on an image pick-up area thereby allowing the user to view the scene to be photographed on the display screen, when the phase difference auto focusing is performed, a reduction in the operational speed may be experienced due to the shutter lag resulting from the necessary movements of the main mirror and the sub mirror. Thus, improved configuration and/or operation of the main mirror and sub-mirror arc thus desired.